The invention relates in general to ion sensors and more specifically to an apparatus, system and method for monitoring an ion concentration of a measured fluid.
Wideband ion sensors are used to measure the concentration of particular ions within a fluid where the fluid may be a gas or liquid. A popular use of wideband gas ion sensors includes using oxygen sensors to determine on oxygen concentration within a gas mixture. Other examples of gas ion sensors include nitrogen sensors that sense gaseous oxides of nitrogen. Many conventional combustion engines utilize oxygen sensors for determining the air to fuel mixture of the exhaust of the combustion engine. Conventional internal combustion engines typically incorporate electronic fueling control using computing devices, such as Electronic Control Units (ECU), that meter fuel into the engine intake depending on engine intake airflow. Typically, the volume of fuel is regulated such that emissions are minimized and all of the fuel is completely burned. The theoretical ratio of air to fuel for complete combustion is 14.7 by weight for gasoline, called the stoichiometric ratio. Theoretically, all available fuel combines with all the intake air at the stoichiometric ratio. The unit Lambda (λ) is often used to represent the quotient of actual air to fuel ratio over the region near the stoichiometric ratio. Conventional electronic fueling systems typically include an oxygen sensor in the exhaust that measures the oxygen concentration of the exhaust. These oxygen sensors act as fuel cells that create an output voltage by combining unburned hydrocarbons in the exhaust with atmospheric oxygen. This results in a lambda/output transfer curve where a λ of 1.0 corresponds to an output voltage of 0.45V. Using the oxygen sensor, the fueling control system regulates the fueling such that the resulting lambda is 1.0 at medium load conditions using a feedback loop. The transfer curve of a typical oxygen sensor is very steep where λ is equal to 1.0, however, and significant variations in output voltage occurs for slight variations in λ. Accordingly, the measured voltage cannot be used to measure other λ values. At high load conditions, a typical internal combustion engine produces maximum power at lambda values <one (0.75 to 0.85). Conventional ECU systems operate in an ‘open loop’ mode under these conditions where the volume of injected fuel is derived solely from pre-stored maps that relate intake air mass to fuel mass without feedback. Because engine aging and production variations change the actual air fuel ratio of the engine, these pre-stored conditions are not always correct for the particular engine. As a result, conventional systems are limited in that severe inefficiencies can occur at high load conditions. Many other wideband ions sensors experience similar drawbacks.
Some recent developments in engine technology have resulted in ‘lean-burn’ systems that operate at lambda ratios greater than 1 (up to 1.1) to minimize fuel consumption and further minimize emissions using special catalytic converters. Because ordinary lambda sensors are not usable in these lambda regimens, a ‘wide-band’ or Universal Exhaust Gas Oxygen (UEGO) sensor has been developed. UEGO sensors combine a small measuring chamber having an orifice open to the exhaust stream, a standard oxygen sensor (Nernst cell), and a pump cell. The pump cell is a solid-state device of porous ceramic that allows oxygen to move between the atmosphere and the measuring chamber. The direction and magnitude of the current through the pump cell (often referred to as the pump current) determines the direction and flow rate of oxygen ions. In conventional systems, an active feedback loop is incorporated such that the voltage at the oxygen sensor portion of the device is held at the stoichiometric voltage. The pump current can then be used to determine the λ value over a wide range of ratios up to the ratio for free air.
FIG. 1A is graphical illustration of a typical relationship between the pump current and Lambda (λ). As shown in FIG. 1A, the resulting curve of pump current vs. lambda value (λ) is non-linear. Although the curve shape does not vary, manufacturing tolerances in the sensors result in different magnitudes of pump current vs. lambda (λ) (i.e. the curve shifts). Attempts to compensate for the variations include incorporating a calibration resistor in the connector to the measuring cell sensor. Unfortunately, this attempted solution does not address all of the variations. Barometric air pressure and exhaust pressure also influence the lambda/pump current relationship. Accordingly, the outputs of these sensors are not accurate. It is therefore desirable to have a measurement method for oxygen sensors that is self-calibrating and self-compensating for all the above variations.
The pump current vs. lambda curve is also highly temperature dependent. Typical UEGOs contain a heater element that maintains the sensor at the desired operating temperature. The temperature coefficient of the heater element is the quotient of change in resistance (ΔR) to the change in temperature (ΔT). Conventional techniques use the positive temperature coefficient of the heater element to regulate input by operating the element at a constant voltage. Because the temperature coefficient, ΔR/ΔT, is fairly small at the operating temperature, the resulting temperature regulation is not very precise. Depending on the sensor, the pump cell impedance, the Nernst cell impedance, or both have a much bigger temperature coefficient, ΔR/ΔT, and would, therefore, allow more precise temperature control. It would be more advantageous to control the temperature of the pump cell. Unfortunately, at lambda values near 1, the pump current is very small or equal to zero and the pump cell impedance can not be accurately measured on a low current. The Nernst cell is typically physically bonded to the pump cell and, therefore, the temperature of the Nernst cell and the pump cell differ by a small amount. In order to measure the Nernst cell impedance, a known fixed current or known fixed voltage have to be impressed on the Nernst cell and the resulting voltage or current then measured. Alternatively, a small alternating current (AC) voltage or current can be impressed on the Nernst cell and the resulting AC impedance measured. The first method requires stopping the lambda measurement for a period of time and also requires impressing the reverse charge on the Nernst cell to speed up recovery. The second method does not interfere with the measurement but requires low pass filters to remove the AC voltage or current from the measured signal. The filters also remove the higher signal frequencies which results in an inability to detect short transient responses. Both methods measure the temperature of the Nernst cell, not the pump cell. During operation, a temperature gradient between the pump cell and the Nernst cell may occur and some temperature control errors may result. Therefore there is a need for precise pump cell temperature control while measuring lambda without resorting to complicated circuitry to remove measurement artifacts.
Further, conventional fuel metering techniques result in significant pollution during the warm up period of the oxygen sensor. In conventional systems where UEGO sensors are used, a precise operating temperature must be attained before the UEGO output value is reliable. This increases the time the fuel injection system runs in ‘open loop’ without knowledge of actual air-fuel ratio. As a result, the time the engine creates uncontrolled warm-up pollution is dependent on the sensor warm-up time. Therefore, there also exists a need for an apparatus, system and method for measuring an oxygen concentration which minimizes the time before a reliable value is produced by the sensor.
Current wideband ion sensors such as wideband oxygen sensors (WBO2 sensors) combine a Nernst cell reference sensor and a pump cell in single package. A Nernst cell is an electrochemical cell that produces a voltage that is nonlinearly proportional to the difference in partial pressure of a measured gas between electrodes of the cell. In a typical oxygen sensor application, the electrodes are exposed to atmospheric air on an electrode on one side of a measuring chamber and to an exhaust gas of an internal combustion engine on the other electrode. A voltage is created by oxygen ions migrating through the solid electrolyte material of the cell. The pump cell is a Nernst cell where oxygen ion flow through the cell is forced by an electrical current. If the current flows in one direction, oxygen ions are transported from the outside air into the sensor. If the current is reversed to the other direction, oxygen ions are transported out of the sensor to the outside air. The magnitude of the current determines the number of oxygen ions that are transported each second.
The Nernst voltage is a voltage created as result of electrochemical reaction in the cell. The cell acts basically as a fuel cell. The Nernst voltage is created by the difference in oxygen partial pressure between the two electrodes of the cell. The Nernst equation describes it:Voutput=(R*)(T)/(n)(F)*ln [(Po,air)/(Po,exh)]
where,
Voutput=O2 sensor's output voltage (0 to 1.0 volt is a typical range)
R*=Universal Gas Constant=8.3143 [Joule/gram-mole*K]
T=Temperature of the exhaust gas [Deg K]
n=number of electrons involved in the reaction=4 in the NBO2 case
F=Faraday constant=96,480 [Coulomb/gram-mole]
Po, air=Partial pressure of O2 in the atmosphere [Pascals]
Po, exh=Partial pressure of O2 in the exhaust gas at temp [Pascals].
In conventional systems, both the Nernst cell and the pump cell are mounted in a very small measuring chamber open with an orifice (diffusion gap) to the exhaust gas. During a rich condition, there is little or no oxygen and relatively high levels of oxidizable combustion products within the measuring chamber. In rich conditions, the WBO2 controller regulates the pump cell current such that just enough oxygen ions are pumped into the chamber to consume all oxidizable combustion products. This action basically produces a stoichiometric condition in the measuring chamber. In the stoichiometric condition, the Nernst reference cell produces 0.45V. In a lean condition where there is excess oxygen, the controller reverses the pump current so that all oxygen ions are pumped out of the measuring chamber and a stoichiometric condition returns. The pump cell is strong enough to pump all oxygen out of the measuring chamber even if the chamber is filled with free air.
The task of the WB controller in conventional systems, therefore, is to regulate the pump current such that there is never any oxygen nor oxidizable combustion products in the measuring chamber. The required pump current is a measure of the Air/Fuel ratio. Conventional wideband sensors, however, are difficult to produce because multiple cells are combined in a small package. Also, the small orifice to exhaust gas is susceptible to contamination or blockage by exhaust particles limiting performance of the sensor. In addition, conventional wideband sensors exhibit a delay between Nernst reference cell output and changing pump cell current because of the physical separation between the two devices. Accordingly, an improved ion sensor is needed.
In addition, conventional oxides of Nitrogen (NOx) sensors are implemented using with a Zirconium oxide (ZrO2) sensor. Conventional ZrO2 sensors use Platinum (Pt) electrodes to detect the O2 content of the gas to be measured. Pt electrodes do not have the capacity to measure NOx, because Nitrous oxide compounds are not disassociated by Platinum (Pt) alone. Alloys of Rhodium and Platinum, however, can be used to disassociate the nitrous oxide compounds. A sensor with an Yttrium stabilized zirconium-oxide electrolyte will produce an output voltage that is proportional to the difference in partial oxygen (O2) pressure between the electrodes if the sensor is operating at the appropriate temperature. When one electrode is exposed to air and the other electrode exposed to exhaust gas, the output voltage follows the Nernst relationship. When an electrical current is passed through a cell formed in this way, the cell acts as oxygen pump, where the oxygen current (in moles/second) is proportional to the electrical current. Sensors where the exhaust side electrode is constructed from a Pt—Rh alloy can also disassociate nitrous oxide compounds. Accordingly, the Nernst Voltage can be represented by:Voutput=(R*)(T)/(n)(F)*ln [(Po,air)/((Po,exh)+(Pn,exh)]
where,
Voutput=O2 sensor's output voltage (0 to 1.0 volt is normal range)
R*=Universal Gas Constant=8.3143 [Joule/gram-mole*K]
T=Temperature of the exhaust gas [Deg K]
n=number of electrons involved in the reaction=4 in the NBO2 case
F=Faraday constant=96,480 [Coulomb/gram-mole]
Po, air=Partial pressure of O2 in the atmosphere [Pascals]
Po, exh=Partial pressure of O2 in the exhaust [Pascals]
Pn, exh=Partial pressure of NOx compounds in the exhaust [Pascals]
The NOx is decomposed at the Pt—Rh alloy electrode into N2 and O2 which causes a local increase in the O2 concentration at the Pt—Rh electrode. The local increase is represented by Pn. Accordingly, the partial pressure from NOx contributes to the relationship. Conventional NOx sensors, however, are limited in that when used in lean burn engines, such as diesel engines, the leftover partial pressure of O2 in the exhaust is very high compared to the partial pressure of NOx compounds. For example, the leftover partial pressure of O2 in the exhaust is typically in the single digit to multi digit percentage range while the partial pressure of NOx compounds is in the parts per million (ppm) range. Therefore, there is also a need for an ion sensor that extracts the NOx content of the exhaust independently to the O2 content.
FIG. 1B is a block diagram of a conventional NOx sensor. The gas to be measured (measured gas) 102 is received through a primary diffusion gap 104 into a first measuring chamber 106. A first pump cell 108 pumps oxygen ions from the first measuring chamber 106 either to atmospheric air or to the surrounding exhaust gas until the remaining gas in the first measuring chamber 106 has a relatively low oxygen concentration. A portion of this oxygen-reduced gas diffuses through a secondary diffusion gap 110 into a secondary measuring chamber 112. A second pump cell 114 in the secondary measuring chamber 112 includes an electrode 116 consisting of a Platinum (Pt) and Rhodium (Rh) alloy exposed to the gas in the second measuring chamber 112. The Rhodium in this alloy has catalytic properties that disassociate the Nitrous Oxides (NOx) in the measurement gas within the second measuring chamber 112 into Nitrogen (N2) and oxygen (O2). As a result, the oxygen (O2) concentration in the second measuring chamber 112 increases slightly. A constant voltage is applied to the secondary pump cell 114 and the current through that pump cell 114 is measured. The NO2 content measurement is based on the current through the secondary pump cell 114. An oxygen measuring cell 120 provides the feedback for the pump cell to regulate the pump current in order to maintain a very low O2 concentration, while not allowing the concentration to decrease to the point where the voltage across the pump cell leads to electrolytic decomposition of the ZrO2 solid electrolyte destroys the pump cell. This conventional method, however, is limited in several ways. The measured current is relatively small (within the nano-Ampere range) and, as a result, is extremely susceptible to electromagnetic noise contamination. Further, such conventional sensors are difficult to manufacture due, at least partially, to multiple diffusion gaps. Also, the gas concentration differences between the first measuring chamber 106 and second measuring chamber 112 are very small. Accordingly, the diffusion flow through the second diffusion gap 110 is significantly delayed resulting in a very slow response time of the sensor.
Therefore, in addition to the needs described above for wideband and NOx sensors, there is a need for a NOx sensor that is easier to manufacture with increased performance.